To make optimal use of the bandwidth available in a fiber based network, it is desirable to use as many separately detectable wavelength channels as possible and to encode data onto each wavelength at as high a speed as possible. One problem with lasers that deliver multiple channels is that the cavity length for many of them is long, making direct modulation above 1 GHz impractical due to the round trip time of a photon in the cavity. A possible solution is to use an output power tap on the main laser cavity and to modulate this output while the main laser cavity runs in continuous wavelength mode. (see C. H. Joyner et. al. "An 8 channel digitally tunable transmitter with electroabsorbtion modulated output by selective-area epitaxy" IEEE Phot. Tech. Let. vol. 7, no.9, September 1995 pp. 1013-1015 or the pending patent application entitled "Improved tunable transmitter with Mach-Zehnder Modulator," Ser. No. 09/016,176, filed on Jan. 30, 1998 by C. H. Joyner. In both of the above cases the modulator was external to the laser cavity.
For increased modulation rates at lower drive voltages, it is desirable to integrate a modulator capability into a laser transmitter and to change from absorptive modulation to phase modulation.